This invention relates to anisotropically electroconductive or so-called Z-axis adhesives. The invention is more particularly concerned with novel electroconductive bodies or particles for use in such adhesives and with adhesives incorporating such particles.
Z-axis adhesives are well known and have been used commercially in the electronics industry for a number of years. The underlying concept of these materials is really quite simple. Basically, a carrier containing a dielectric resin adhesive is loaded with a minor amount of electrically conductive, non-bonding particles (usually metal spheres) which ultimately serve to provide the desired electrical connections. The carrier may be in the form of a solvent-containing paste, in which the particles are randomly dispersed. The paste may be sandwiched as a thin coating between opposed surfaces of electronic components to be joined and to be electrically connected at selected locations. Heat and pressure are then applied to the sandwich such that the dielectric adhesive is caused to conform to the topography of the opposed component surfaces and to bond them adhesively.
Electrical connections are established by pressure contacts of the non-bonding conductive particles and the electrical contact pads on the opposed component surfaces. The loading of conductive particles in the carrier is kept low enough as to avoid the formation of continuous electrically conductive paths through the particles in the adhesive plane (X,Y plane).
Pastes as just described are also commonly formed into thin Z-axis films, as by casting. Such films, which thus consist of a random dispersion of conductors in a sheet-like carrier of dielectric adhesive, are used in an analogous manner to pastes in order to effect bonding of components.
Reliability of electrical connections formed by non-bonding conductive particles requires that the particles be maintained under stress by the dielectric adhesive. In practice, however, the connections are subject to failure due to such factors as stress relaxation of the dielectric adhesive film and corrosion of metalized surfaces. Differential thermal coefficients of expansion of materials in the circuit assembly may also lead to deterioration of the electrical connections.
Z-axis adhesives employing solder and solder-coated particles, which provide a bonding effect, are also known. Solder, however, is compatible with only a limited range of contact surface materials, and contact surfaces must be cleaned of oxides by fluxing. Solder also requires the application of relatively high temperatures, typically about 220.degree. C., to effect bonding. Such high temperatures are undesirable in many applications. Liquid crystal displays, for example, cannot tolerate such temperatures. Nor can many low cost circuits, such as those made on Mylar (polyester), tolerate soldering. Solders used in the electronics industry also commonly include lead, which has been linked to various health and environmental problems.